What are common errors when performing calculations with variables in PHP?

Common errors when performing calculations with variables in PHP include not properly declaring or initializing variables, using incorrect variable names, and mixing data types in calculations. To avoid these errors, ensure that all variables are declared and initialized before performing calculations, use correct variable names consistently, and pay attention to data types when performing mathematical operations.

// Example of properly declaring and initializing variables for calculations
$num1 = 10;
$num2 = 5;

// Performing addition with correctly declared variables
$sum = $num1 + $num2;
echo "The sum is: " . $sum;